Schematic illustration of the mechanism of digital and sustained oscillation. The upper curve is a bifurcation diagram, p53 versus concentration of activated ATM. The lower curve is the dependence of activated ATM on the damage levels. The parameter k5 donates basal unrepaired DNA damage ([ATM*]: activated ATM. The limit point for the mutant switch is 7.11, for non-mutant switch 9.10).